The present invention relates to a loudspeaker having a narrow directivity and more particularly to a horn loudspeaker.
The loudspeaker is roughly divided into an electrodynamic speaker using magnetic flux and electromagnetic force caused by electric current, and an electrostatic speaker using electrostatic force between electrodes in accordance with the operating principle of the speaker.
A directional characteristic of the speaker is usually indicated by a sound pressure frequency characteristic at an angular position of 30.degree. or 60.degree. with respect to the forward axis of the speaker. The characteristic is considered as an important factor in sound reproduction performance of a stereo system. More particularly, since the best listening position for ordinary stereo reproduction is a location where the distances to the right and left speakers are substantially equal, the position is angularly deviated from the forward axis of each speaker by some degrees. Hence, when the speaker has a poor directional frequency characteristic, sound is not sufficiently reproduced in the middle and high frequency ranges and inferior in life-like performance.
On the other hand, a speaker with a narrow directivity is preferable for radiating sound only in a predetermined area.
FIG. 4 shows an example of a conventional horn speaker, which comprises a radiating element 1 and a horn 2. The sound wave from a diaphragm provided inside the radiating element 1 propagates through the horn 2 instead of immediately diffusing. The directivity of the sound depends on the size of the opening of the horn 2. Compared to a direct-radiator speaker, the directivity of the horn speaker can be easily narrowed.
Referring to FIG. 5, a speaker S is provided at a front portion thereof with a cabinet 3 made of a sound insulating material having a cylindrical sound absorptive board 6 at a center therein. A sound absorptive element 5 comprising a sound absorptive material 4 is provided in the cabinet 3. The cylindrical sound absorptive board 6 is so disposed as to be parallel to the axis X of the speaker. The sound waves off the axis X are absorbed by the absorptive material 4 to obtain a narrow directivity.
However, in order to obtain the narrow directivity even in a low-frequency range, the diameter of the opening of the horn 2 must be increased, which causes the enlarging of the speaker.
Moreover, sound waves which are reflected at the opening of the horn affect the sound waves propagating along the axis of the speaker. Hence, as shown in FIGS. 6 and 7, showing frequency responses of the speakers shown in FIGS. 4 and 5 respectively, peak dips of the sound pressure appear in the characteristics.
Furthermore, in the speaker shown in FIG. 5, the sound waves reflected by the cabinet 3 affect the sound waves on-axis, causing lobes in the characteristic lines as shown in FIG. 7. The lobes and peak dips generated in the reproduced sound waves deteriorate the reproduction characteristics of the speaker.